map

Override with your map implementation. This overload is given a single
local input Chunk. It is meant for map/reduce jobs that use a
single column in a input Frame. All map variants are called, but only one is
expected to be overridden.

reduce

Override to combine results from 'mrt' into 'this' MRTask. Both 'this'
and 'mrt' are guaranteed to either have map() run on them, or be the
results of a prior reduce(). Reduce is optional if, e.g., the result is
some output vector.